The present invention relates to shafts and, more particularly, to tubular shafts in which induced modes of vibration subjected upon the shafts are carefully controlled.
By way of example, one form of tubular shaft which is contemplated as on a golf club.
Golf clubs are typically assembled to include a club shaft having selected performance characteristics and a club head having matching or complementary performance characteristics. A number of factors must be considered in the design of the club head and the club shaft to assure optimal performance when hitting the golf ball. Many of the design factors for both the club head and the shaft are related to dimensional and static mass characteristics. For example, principal club head design parameters include the overall mass, the club face angle and surface characteristics, the dimensional envelope, and the location of the center of gravity. Similarly, principal club shaft design parameters include the length of the shaft, its diameter, the change in shaft diameter with length, the overall mass, and its flex characteristics.
Additionally, attention in the design and manufacture of golf club shafts has been focused on the flex and torsional damping characteristics of the golf club shaft since it has been discovered that the so-called damping characteristics have a direct and primary role in determining the "feel" of the golf club during impact.
With regard to use of the golf club, the golf club stroke can typically be divided into separately defined portions, namely the takeaway, the backswing, the downswing, impact, and the follow through. During the takeaway, the golf club is taken back to set up that portion of the swing generally known as the downswing somewhat to cause the club head to lag behind the shaft. As the downswing is initiated, the direction of the shaft movement and that of the club head is reversed with the club head lagging or following the shaft. The amount of club head lag is a function of the shaft stiffness and the torque applied to the shaft during downswing. Since the club head is on the distal end of the shaft during the downward acceleration, the club head accelerates more quickly than any other point along the shaft and, for most shafts, the club head will lead the shaft at some point in the downswing prior to impact. Because of the flexibility of the shaft, the club head has downswing flight characteristics somewhat akin to an object in tethered flight.
Among the consequences of these club head flight characteristics prior to impact are small changes in the angular relationships of the club face in relation to the longitudinal axis of the unflexed shaft. These angular changes affect the engagement of the club face with the ball at impact and, therefore, the subsequent flight path of the ball. During impact, the golf ball is compressed to define a contact area between the club face and the compressed surface of the golf ball through which a portion of the momentum of the club head is imparted to the ball. The time of actual contact between the club face and the golf ball is generally on the order of approximately 450 to 600 microseconds. As a result of the club head contacting the ball at impact, traverse and torsional vibrational waves are induced which travel upwardly along the length of the shaft toward the grip. For purposes of the present invention, "torsional vibration" is defined as the oscillatory displacement about the longitudinal axis of the shaft and "transverse vibration" is the oscillatory displacement occurring perpendicular to the longitudinal axis of the shaft.
As a consequence of the momentum transfer at the club face to the ball during impact, the shaft is flexed rearwardly so that the club head again lags behind and follows the shaft. After impact and during follow through, the club head oscillates between lagging and leading positions as a consequence of the natural frequencies of the shaft, these oscillations including several modal orders above the lowest order.
In an effort to enhance the "feel" of the golf club, golf shafts have been developed which are formed of composite fibers in which the shafts are fabricated from oriented non-metallic fibers, i.e., graphite, boron, glass, etc., in an epoxy matrix. For example, graphite shafts typically include an inner lamina fabricated with fibers that are oriented at complementary angles to the longitudinal axis of the unflexed shaft, e.g., +45.degree. and -45.degree., to provide a measure of torsional stiffness, and an outer lamina fabricated with fibers that are substantially parallel to the longitudinal shaft axis to provide longitudinal stiffness. Typically, graphite shafts and composite shafts in general, have a somewhat "damped" feel wherein the effects of high vibrations along the shaft are less traumatic. The longitudinal stiffness can be controlled by varying the size and number of longitudinal fibers, and the torsional stiffness can be varied by controlling the angularly oriented fibers to provide a measure of independence between the two characteristics, sometimes it can be difficult.
In an effort to achieve a better "feel" still further developments in the art have focused on selectively damping golf club vibrations by controlling vibrational frequencies through the use of devices disposed along various lengths of the golf club shaft. Typically, such devices have included disposing sleeve-like members including a first layer of elastomeric material and a second layer of a metallic material about the inner or outer surface of the shaft as disclosed in U.S. Pat. No. 5,249,119 which issued Mar. 15, 1994, to Vincent et al.
The need for golf club shafts which offer isotropic material properties and which posses the internal damping characteristics seen, for example, in composite golf club shafts is readily apparent. By "isotropic," it is meant that the shaft to which the dampening material is applied will essentially have the same strength and elastic properties in all directions (i.e. similarities along the length of the shaft with regard to the modulus of elasticity, modulus of rigidity and Poisson' ratio). As a consequence of this isotropic effect, shafts and, more particularly, steel shafts are more consistent over a spectrum or set and allow for a tighter dispersion of shots.